Not applicable.
The invention relates to a method for sealing the coating of a roll and particularly an intermediate layer thereof, particularly of a press roll such as the center roll of a press or the back-up roll of an extended-nip press roll, or of a calender roll used in a pulp, paper, board or finishing machine, and, further, to a roll and a coating for a roll. The coating of a roll, particularly a thermally sprayed coating, can be improved in terms of its corrosion resistance and impermeability by virtue of applying a sealing to the coating.
In paper and board machines, rolls with ceramic and cermet coatings have by and large replaced granite rolls used earlier in the art. With rolls with ceramic or cermet coatings, corrosion of various degrees occurs, in particular in more severe conditions, causing corroding of the roll, delamination of the coating and wear of the roll so that it gradually becomes unusable. A coating made on a roll by way of thermal spraying always remains somewhat porous, thus allowing gradual penetration of moisture through the porous surface toward the inner metallic layers thus causing corrosion and so the protection offered by the coating against corrosion is insufficient. Process conditions in the paper and board industry have continually become more and more severe and demanding on the equipment, among other reasons, due to the trend toward enhanced circulation of process water for environmental protection and change of the process environment in a more corrosive direction.
There have been attempts to improve the corrosion resistance of rolls, e.g., by applying metal or cermet layers between the ceramic or cermet coating and the roll mantle. Such methods are described in FI Patent Nos. 86,566, 82,094 and 84,506. From FI Patent Application No. 971,541 is known the application of an impervious intermediary layer under the porous ceramic outer layer, whereby impermeability of the intermediate layer is attained by way of high-speed flame spraying.
Attempts have also been made to seal the outer surface of a ceramic coating. In WO publication No. 97/15719, a technique is disclosed for sealing a ceramic coating by means of an inorganic solution. Therein, a separate adherence-improving layer is made underneath the ceramic coating layer, and the ceramic coating layer itself is formed by a single porous layer, reduction in the porosity of which has been sought by way of applying an inorganic solution on the roll surface after the coating step. In EP Patent Application No. 0,481,321, a roll for a paper machine is described having a synthetic resin or wax applied onto the ceramic coating in order to seal its pores. Conventional epoxy resins are mentioned as an example of suitable synthetic resins. In FI Patent No. 90,098, a method for coating a roll is described, in which noncontiguous areas of polymer are applied to the roll surface, whereby the ceramic coating of the roll outer surface is impregnated only by a partial length of the roll. Accordingly, all cited publications aim at sealing the pores of the outer surface of the ceramic coating by means of a sealing agent thus preventing corrosive substances from penetrating under the coating.
The latter type of sealing systems performed according to the prior art are problematic in that the material used as the sealing agent changes the properties of the roll surface and, on the other hand, the sealing agent is rapidly lost from the surface thus again changing the surface properties. This makes the control of the runnability of the paper machine more difficult and increases the risk of web breaks. Obviously, there is an imminent need for an impervious roll coating and a method for sealing the surface of a roll so that corrosion can be prevented and, on the other hand, the original desirable properties of the ceramic coating can be kept unchanged.
The object of the present invention is to provide an impervious coating for a roll used in a paper or board machine and a method for sealing the coating of a roll and, further, a roll having an impervious surface.
The shortcomings of conventional methods known in the art for sealing surface coatings in paper or board machines can be avoided or decisively reduced by virtue of the method described in the following. The invention is based on intermediate sealing of a thermally sprayed coating such as a ceramic coating, whereby the sealing step is carried out prior to the application of the outermost surface layer. Sealing of the coating is performed so that, during the surfacing of a roll, the application of the coating is stopped when a given thickness of coating has been reached and subsequently the sealing treatment is carried out, or alternatively the coating application may be continued and a plurality of such sealing treatments can be carried out. After the last sealing treatment, the coating process is continued up to the final coating thickness. Herein, the sealing agent remains entrapped under the final ceramic coating layer proper, which acts as the functional coating, and thus prevents substances corrosive to the roll frame from reaching the metallic inner part of the roll. The sealing of the intermediate layers may also be performed on a metallic adhesion or corrosion protection layer applied to the outer surface of the metallic roll or, alternatively, to a selected area thereof, and/or said adhesion or corrosion protection layer is next covered with a ceramic layer of a given thickness that is sealed to form an intermediate layer in the above-described manner, whereupon the ceramic coating can be prepared by spraying to its final thickness. After each intermediate layer sealing step, the surface is subjected to an adhesion-improving complementary treatment prior to the spraying of the next coating layer.